This invention relates to a fluid pumping system and, more particularly, to a fluid pumping system adapted for use with a natural gas dehydrating system of the type employed at a gas well head to remove water from a well stream composed of a mixture of gas, oil and water.
Examples of such gas dehydrating systems are disclosed in U.S. Pat. Nos. 3,094,574; 3,288,448; and 3,541,763; the disclosures of which are specifically incorporated herein by reference. In general, such systems comprise a separator means for receiving the gas-oil-water mixture from the well head and separating the oil and water liquids from "wet" (water vapor laden) gas; and a water absorber means, which employs a liquid dehydrating agent such as glycol, for removing the water vapor from the wet gas and producing "dry" gas suitable for commercial usage. The glycol is continuously supplied to the absorber means in a "dry" low water vapor pressure condition and is removed from the absorber means in a "wet" high water vapor pressure condition. The wet glycol is continuously removed from the absorber means and circulated through a treater means such as a reboiler means for removing the absorbed water from the glycol to provide a new supply of dry glycol. The glycol reboiler means usually comprises a still column associated with a gas burner for heating the wet glycol to produce hot dry glycol by removing the absorbed water and any entrained gas by vaporization. The hot dry glycol passes through a heat exchanger, where the hot glycol is cooled and the incoming wet glycol is heated, to a dry glycol storage tank. A glycol passage means is provided to enable passage of wet glycol from the absorber means to the reboiler means and to pump dry glycol from the storage tank to the absorber means. In most cases, the absorber means operates at a relatively high pressure (usually 50 psig to 2000 psig) while the reboiler means which removes the absorbed constituent from the wet glycol operates at a relatively low pressure, e.g., atmospheric pressure. The circulation rate of glycol required to achieve dehydration is a function of the natural gas flow rate, temperature and pressure. Since the dehydrating systems are continuously operated at a well site without continuous monitoring by operating personnel, reliable continuous operation of the glycol pump is of critical importance. In addition, it is highly desirable to use energy sources available at the well site for operation of the pump with maximum efficiency and minimum energy loss.
The present invention provides a new improved glycol pumping system which is operated by an available energy source other than the saleable dry natural gas at the well head; which may be operated at relatively low speeds and pressures without stalling; and which is automatically continuously operable under a wide range of operating conditions.
Prior art glycol pumping devices are described in Kimmel, U.S. Pat. No. 2,990,910; Gerlach et al., U.S. Pat. No. 4,431,433; and Gerlach et al., U.S. Pat. No. 4,402,652 which are hereby incorporated by refernece for all that is contained therein.
Each of the pumping devices described in these patents and the device of the present invention include a pump section and a motor section. The function of the pump section is to pump dry glycol from a treater or reboiler back into the absorber vessel. The function of the motor section is twofold. It provides power to drive the pump section and also meters the flow of wet glycol out of the absorber and back into the treater or reboiler.
Ideally, if there were no friction or flow pressure losses in the pumping and motor functions, the wet glycol returned from the absorber to the treater means would provide sufficient energy to power the pump which pumps dry glycol from the treater back into the absorber. In fact, ideally, the motor would have an excess of energy since the wet glycol and dissolved gas therein metered out through the pump motor is greater in volume, generally at least 3% greater, than the dry glycol pumped back into the absorber. Ideal pumping conditions cannot be achieved since significant flow losses and friction are present in an actual physical system. Prior art pumping systems have utilized well gas to increase the pumping energy of the system to make up for these friction and flow energy losses.
The device of U.S. Pat. No. 2,990,910 derives the power for the motor section entirely from the fluid metered out of the absorber and returned to the treater. The excess energy required to overcome friction, etc., is obtained by making the volumetric displacement of the motor section greater than the volumetric displacement of the pump section. The amount of volumetric imbalance is typically 25% to 50%. This imbalance requires a certain volume of natural gas to be metered out of the absorber along with the wet glycol. Generally, this gas is subsequently wasted by being vented into the atmosphere. The environmental and economic impact of this waste is considerable. For example, a gas well producing 10 million cubic feet per day from a 1100 psi well using a pump of the type described in U.S. Pat. No. 2,990,910 may waste about $25,000.00 worth of gas in one year in the pumping operation.
The pumps described in U.S. Pat. Nos. 4,431,433 and 4,402,652 require less gas comsumption than the pump of U.S. Pat. No. 2,990,910. Pumps of this type utilize a two stage motor section. The first stage is powered by wet glycol metered from the absorber by a cylinder whose volumetric displacement is about 8% greater than the pump section volumetric displacement. A reduction in imbalance is thus achieved which results in a significant reduction in gas consumption. The second stage motor section is gas powered but no significant gas waste results because the gas powering this stage is subsequently entirely directed to the burner. However, the pump devices of U.S. Pat. Nos. 4,431,433 and 4,402,652 are considerably more mechanically complex than that of the device of U.S. Pat. No. 2,990,910.
One objective of the present invention is to achieve a significant gas consumption reduction over the device of U.S. Pat. No. 2,990,910 without resorting to the complexity of the devices of U.S. Pat. Nos. 4,431,433 and 4,402,652.
Another objective of the present invention is to provide a pump/pump motor which can be easily and efficiently maintained in the field without return to a central shop area for minor overhaul work.
Another objective of the present invention is to provide a pump/pump motor in which the volumetric imbalance areas may be rapidly and easily changed to adjust for changing well or delivery conditions.
Another objective of the present invention is to provide a pump/pump motor which will reliably restart when automatically stopped and started on a well which is periodically "stop clocked".
Another objective of the present invention is to provide a pump/pump motor which, because of reduced friction and other features, will pump over a wider pressure range and run at much lower speeds without stalling, than existing units.